In the oil industry, electromagnetic (EM) induction surveys are used to map the electrical conductivity of geologic formations between boreholes and/or radially away from a single wellbore. The latter, usually referred to as induction logging, has been in routine use for over fifty years.
The various types of induction surveys typically share many commonalities. A transmitter, usually a multi-turn coil of wire, carries an alternating current of frequency ω (radians/sec). This creates a time-varying magnetic field in the surrounding formation that in turn, by Faraday's law, induces an electromotive force (emf). This emf drives currents in the formation that are basically proportional to the formation conductivity. Finally, a receiver is positioned either in the same hole as the transmitter, in another hole, or on the surface (land or seafloor), and measures the magnetic field arising from the transmitter and the secondary or induced currents in the formation. Conventional induction logging always uses a combination of multiple receivers and/or multiple transmitters connected in series so as to cancel the mutual signal in air. In general, a theoretical model for a logging system embedded in a formation of arbitrary resistivity is used to match or interpret the received signals. In some applications, the absolute value of the average formation resistivity is not as important as the ability to map variations of resistivity within the formation. To determine this spatial variation of formation resistivity, the surveys typically involve placing the transmitter at multiple locations in the hole and measuring the fields at multiple receiver locations for each transmitter location.
Surface to borehole EM (as well as borehole to surface configuration) surveys have been described in detail in applications commonly owned with the present application, including:                U.S. patent application Ser. No. 12/719,730 filed Mar. 8, 2010, entitled “Electromagnetic Detection of Base of Salt While Drilling”,        U.S. patent application Ser. No. 12/581,947 filed Oct. 20, 2009 entitled “Detecting Electrical Current in a Magnetic Structure”,        U.S. patent application Ser. No. 12/641,944 filed Dec. 18, 2009, entitled “Correction Factors For Electromagnetic Measurements Made through Conductive Material”,        U.S. patent application Ser. No. 12/641,898 filed Dec. 18, 2009, entitled “Attenuation of Electromagnetic Signals Passing Through Conductive Material”,        U.S. patent application Ser. No. 12/603,053 filed Oct. 21, 2009, entitled “Electromagnetic Logging Between Borehole and Surface”, and        U.S. patent application Ser. No. 12/405,214 filed Mar. 16, 2009, entitled “Casing Correction in Non-magnetic Casing by the Measurement of the Impedance of a Transmitter or Receiver”.        Each of the above shares a common assignee with the present application, and is incorporated herein by reference in its entirety.        
Water encroachment in horizontal wells is a well known problem. An induction logging system that will measure, monitor, a waterfront approaching a producer would provide useful information in management of production and reservoir. Typically electrical resistivities of water and hydrocarbons are significantly different, and electromagnetic (EM) measurements are very sensitive to these resistivity changes.
Crosswell EM measurements, where an EM source is placed in one well and an EM receiver is placed in another, have been used to map resistivities between two wells and monitor movement of water. EM techniques are ideal for monitoring recovery/production processes compared with seismic surveying techniques, because of the large-scale fluid and heat flow. The accuracy of cross-well surveys decreases as the distance between wells is larger, though under certain circumstances Schlumberger's cross-well system has been proven to determine the resistivity distribution between wells spaced up to 1000 m apart. Heretofore, surface-to-borehole EM surveys have been insufficiently sensitive to deep-seated resistivity changes.